There are various methods for starting huge engines, for example, engines for use in a generator drive or main engines for ships. One starting method is to directly drive the output shaft of the engine using an air motor (air starter) utilizing compressed air. The air motor continues to rotate until a fuel fed to the engine is ignited and the engine can increase its engine speed for itself. After that, the air motor is decoupled from the engine and stops. The starting method using the air motor is disclosed in Patent document 1 as recited below.
Typically, one air motor is provided for each engine, but there are huge engines provided with two or more air motors. As the engine increases in size, the air motor increases in size and number. The capacity (size) of the air motor is determined by an engine rotational torque required to start rotation of the engine in a stopped state, an engine rotational torque required to maintain or increase the engine speed at which a fuel fed to the engine is ignited, and a reduction gear ratio between a drive gear (pinion) mounted on the air motor and a driven gear (ring gear) mounted on the drive shaft of the engine. When the engine is started using two or more air motors, these air motors are operated together from the start of the operation until the end of the operation.    Patent document 1: Japanese Laid-Open Patent Application Publication No. Hei. 2-277962